The present invention relates to electrodes for electrochemical cells, which electrodes exhibit a certain gas permeability for supplying the reaction gases and due to which a uniform distribution of the reaction gases over the entire contact surface to the electrolyte becomes possible.
Electrochemical cells consist of an anode side electrode and a cathode side electrode which serve for making the electric contact, however also for supplying and carrying away the educts and products of the electrochemical reaction. They further consist of an electrolyte situated between the electrodes.
An example for such an electrochemical cell is a polymer electrolyte membrane (PEM) fuel cell. In this case a membrane made of a proton-conducting polymer is used as electrolyte. On both sides of the membrane, either on the membrane itself or on the adjacent gas diffusion electrodes, there is a porous layer which contains the catalysts necessary for the electrochemical reaction. Then, on both sides, there are the so-called gas diffusion electrodes, which serve for supplying the reaction gases hydrogen and air. At the anode side the hydrogen supplied is split into protons and electrons at the catalyst. The protons move through the polymer electrolyte membrane to the cathode side of the fuel cell where they react with the oxygen from the air to form water. The electrons needed for this are supplied from the anode via an external circuit to the cathode. In this circuit they can do electric work.
Document WO 01/54213 describes the use of films made of pressed expanded graphite with so-called gas channels as electrodes in electrochemical cells. These gas channels are uninterrupted openings from one side of the graphite film to the other. They are necessary since a closed film of pressed expanded graphite exhibits only a very low gas permeability so that without these openings it would not be possible to supply a sufficient amount of reaction gases to the catalyst and to the membrane. This document describes electrodes with two plane, parallel surfaces, having such shape even after the openings in the graphite film are formed. In particular, the side towards the membrane or towards the catalyst layer on the membrane, respectively, is plane and does not have any projecting parts or the like. Therefore, the graphite film of the electrodes is everywhere in direct contact with the catalyst layer, with the exception of those areas where one of the openings is in the electrode. However, there is hardly any gas diffusion, from the openings of the electrode, parallel to the membrane, since although the catalyst layer is porous, it is also very thin. This means that hardly any reaction gases can reach those parts of the catalyzed membrane which are covered by the electrode. Thus, the disadvantage of the described electrode is that only a very small part of the available membrane area is actually used, which leads to significant losses of power e.g. in a fuel cell.